Velbel, M. A. 2007. Surface textures and dissolution processes of heavy minerals in the sedimentary cycle: Examples from pyroxenes and amphiboles. Vol 58, Pages 113-150 in M. Mange and D. Wright, eds. Heavy Minerals in Use: Developments in Sedimentology. Elsevier Press, Oxford, UK.
Natural weathering of pyroxenes and amphiboles corrodes them in a crystallographically con-trolled manner, resulting in ubiquitous etch pits on grain surfaces and eventually denticulatedmargins. Most denticles originate at fractures, but others form without fractures, along arrays oflaterally adjacent dislocations. The range of corrosion morphologies is identical betweenpyroxenes and amphiboles. Weathered pyroxenes and amphiboles at numerous locations and in awide variety of weathering environments exhibit the same types and ranges of weatheringmorphologies across a wide range of mineral compositions, regolith types and pedogenic/geo-chemical environments. Similar ranges of dissolution textures also occur on pyriboles (a shortsingle term for single- and double-chain silicates) in modern sediments and on intrastratallydissolved pyriboles in clastic sedimentary rocks.
Mineral grains are exposed to varying physical and chemical conditions as they move throughthe sedimentary cycle. Grain surfaces respond to these changing conditions; thus, surface textureson detrital heavy-mineral grains are used to infer weathering processes in soils and weatheredregoliths, provenance and sedimentary environments, and intrastratal dissolution during burialdiagenesis. The ability to arrive at useful interpretations from sand grain morphology and/orgrain-surface textures is based on specific relationships (which differ among different heavyminerals) between rates of surface-texture modification in a given environment and the length oftime that the grain is exposed to modifying processes in that part of the sedimentary cycle. Forchemically produced features, the timescale for the development of new grain-surface texturesdepends on the geochemical kinetics of the chemical texture-modifying reactions, which are inturn related to the persistence of each mineral in weathering and the sedimentary cycle.Pyroxenes and amphiboles are minerals with intermediate rates of dissolution and intermediatepersistence among heavy minerals; they corrode gradually and survive long enough for the degreeof corrosion to be a scientifically useful soil relative-age and environmental indicator. The sig-nificance of etch pits and other surface textures on heavy minerals in a soil or regolith depends on the rates at which they form in that regolith, and how long the grains have been subject to suchreactions (i.e., the age of the soil) in the weathering environment. Similar relationships are to beexpected in other compartments of the sedimentary cycle.back to index